1. Field of the Invention
The present invention relates to a frame synchronizing signal (a unique word) detecting method, particularly relates to a frame synchronizing signal detecting method used for a data multiplexing transmitter-receiver of achieving the stable establishment of a link by enhancing the detection precision of a frame synchronizing signal before the link of a frame synchronizing signal is established and of preventing non-detection due to noise and disturbance by lowering the detection precision of a frame synchronizing signal after the link of a frame synchronizing signal is established.
2. Description of the Related Art
Generally, a frame synchronizing signal (also called a unique word or a flag) having a specific bit pattern is added to the head of data included in each data packet, when a data multiplexing transmitter-receiver at the transmitting end transmits data, it transmits the data of which a frame synchronizing signal is added to the head of the data string, when another data multiplexing transmitter-receiver at the receiving end receives the transmitted data, it detects the frame synchronizing signal added to the data and frame synchronization with the data multiplexing transmitter-receiver at the transmitting end is achieved using the detected frame synchronizing signal.
A frame synchronizing signal is normally detected by a frame synchronizing signal detector provided to a baseband signal processor in a data multiplexing transmitter-receiver. In this case, when the frame synchronizing signal detector receives data, it searches a frame synchronizing signal having a specific bit pattern in the received data, when the added position of the frame synchronizing signal in a frame is verified by the search, a frame synchronization link is established and afterward, the frame synchronizing signal is extracted based upon the verified added position of the frame synchronizing signal.
FIG. 2 is a block diagram showing one example of the configuration of the main part of a known data multiplexing transmitter-receiver.
As shown in FIG. 2, a data multiplexing transmitter-receiver 20 is provided with an antenna 21, a duplexer 22 formed by a change-over switch, a radio-frequency (RF) transmitter-receiver 23 provided with a radio-frequency (RF) signal transmitter 231 and a radio-frequency (RF) signal receiver 232, a modulator-demodulator (MODEM) 24 provided with a modulating circuit (MOD) 241 that converts a baseband signal to a radio-frequency signal and a demodulating circuit (DEM) 242 that converts a radio-frequency signal to a baseband signal and a frame synchronizing signal detector 251, includes a baseband signal processor 25 that processes a baseband signal, a control panel 26 and a microcomputer and also includes a control unit (CPU) 27 that generally controls the data multiplexing transmitter-receiver 20.
The common terminal of the duplexer 22 is connected to the antenna 21, one selection terminal is connected to the output terminal of the radio-frequency signal transmitter 231, the other selection terminal is connected to the input terminal of the radio-frequency signal receiver 232 and the control terminal is connected to the control unit 27. The input terminal of the modulating circuit 241 is connected to the output terminal of the baseband signal processor 25 and the output terminal is connected to the input terminal of the radio-frequency signal transmitter 231. The input terminal of the demodulating circuit 242 is connected to the output terminal of the radio-frequency signal receiver 232 and the output terminal is connected to the input terminal of the baseband signal processor 25. The baseband signal processor 25 is connected to the control unit 27 and the control panel 26 is also connected to the control unit 27.
The data multiplexing transmitter-receiver 20 having the configuration described above operates as follows.
Data transmission between the data multiplexing transmitter-receiver 20 and a data multiplexing transmitter-receiver on the side of a counterpart (not shown) is set so that the transmitting timing of transmit data and the receiving timing of receive data alternately present themselves by time division multiplexing.
When the data multiplexing transmitter-receiver 20 comes into data transmitting timing, the control unit 27 instructs the baseband signal processor 25 to generate transmit data and switches the duplexer 22 to the side of the radio-frequency signal transmitter 231. At this time, the baseband signal processor 25 generates packet data of a determined format and supplies the packet data to the modulating circuit 241. The modulating circuit 241 modulates the supplied packet data to a transmitting radio-frequency signal and supplies it to the radio-frequency signal transmitter 231. The radio-frequency signal transmitter 231 amplifies the supplied transmitting radio-frequency signal so that the level reaches a transmission level, supplies it to the antenna 21 via the already switched duplexer 22 and transmits it to the data multiplexing transmitter-receiver on the side of the counterpart in a transmitting timing as a radio signal.
In the meantime, when the data multiplexing transmitter-receiver 20 comes into data receiving timing, the control unit 27 instructs the baseband signal processor 25 to prepare for the processing of receive data and switches the duplexer 22 to the side of the radio-frequency signal receiver 232. At this time, when a radio signal including receive data is incoming to the antenna 21, the radio-frequency signal receiver 232 receives the radio signal as a radio-frequency signal for receiving from the antenna 21 via the already switched duplexer 22, amplifies the radio-frequency signal for receiving to a predetermined level and supplies it to the demodulating circuit 242. The demodulating circuit 242 demodulates the supplied radio-frequency signal for receiving to a baseband signal and generates packet data. The packet data is supplied to the baseband signal processor 25 and after it is checked whether the packet data is normal receive data or not by the baseband signal processor 25, data is extracted and is supplied to the control unit 27 and others. When the frame synchronizing signal detector 251 in the baseband signal processor 25 receives data, it searches a frame synchronizing signal having a specific bit pattern in the received data as described above, when the added position of the frame synchronizing signal in the data is verified in the search, a frame synchronization link is established, afterward, the frame synchronizing signal is extracted based upon the verified added position of the frame synchronizing signal and the extracted frame synchronizing signal is supplied to the control unit 27 and others.
Afterward, when the data multiplexing transmitter-receiver 20 comes into data transmitting timing, the same processing as that at the transmitting timing is executed again and afterward, receiving data and transmitting data are also repeatedly executed.
The known data multiplexing transmitter-receiver 20 sets the detection precision of a frame synchronizing signal to detection precision experientially determined (regular detection precision) when the frame synchronizing signal is detected by the frame synchronizing signal detector 251and establishes a frame synchronizing signal link depending upon the set regular detection precision.
The reason why the detection precision of a frame synchronizing signal is set to such regular detection precision is as follows. If the detection precision of a frame synchronizing signal is set so that it is higher than the regular detection precision, there is an advantage in that error synchronization is reduced and the establishment of a stable link can be achieved before a frame synchronizing signal link is established (a link is tried), however, there is a disadvantage in that after a frame synchronizing signal link is established, non-detection due to slight noise superimposed on data and disturbance is often caused and the detection rate of a frame synchronizing signal is deteriorated. In the meantime, if the detection precision of a frame synchronizing signal is set so that it is lower than the regular detection precision, there is an advantage in that the occurrence of non-detection due to slight noise superimposed on data and disturbance is reduced and the detection rate of a frame synchronizing signal is enhanced after a frame synchronizing signal link is established, however, there is a disadvantage in that before a frame synchronizing signal link is established, error synchronization is frequently caused and it takes a long time to establish a link.
The regular detection precision used in the known data multiplexing transmitter-receiver 20 when a frame synchronizing signal is detected is acquired as a result by relieving the disadvantage in the case where the detection precision of a frame synchronizing signal is set so that it is higher than the regular detection precision and the disadvantage in the case where the detection precision of a frame synchronizing signal is set so that it is lower than the regular detection precision, however, these disadvantages are not completely solved.